The present invention relates generally to radio frequency (RF) filters and more specifically to dielectric antenna duplexer and dielectric filter topologies suitable for use in portable electronic devices, such as mobile telephony equipment.
It is well known that state of the art communications systems require high performance filtering devices in order to maximize performance and comply with federal communications laws and standards. These devices are formed to be highly frequency selective by minimizing signal loss within a desired passband and significantly attenuating unwanted signals which reside outside the passband. However, the constraints which are imposed when size reduction of RF filters is desired while maintaining the performance level of such devices makes current filter topologies impractical for many applications. The development of new filter topologies that produce transfer functions with multiple transmission zeroes is one answer to these design constraints. Of course, to realize actual devices, the circuit topologies that implement these transfer functions must have realizable element values for the media in which the devices are constructed.
Current filter topologies, such as the one described in U.S. Pat. No. 5,488,335, employ the technique of resonating the coupling elements between resonant sections of the described device in order to produce finite transmission zeroes. This technique has been used in multi-layer planar circuits but the element values are too large to effectively implement highly selective transfer functions. To the contrary, only relatively non-selective transfer functions may be implemented in this manner, as is exemplified in the response graphs of FIG. 6a and FIG. 6b of U.S. Pat. No. 5,719,539. One qualitative measure of this selectivity is the relative proximity the transmission zeroes to the passband. The device described in U.S. Pat. No. 5,488,335 meets generally performance requirements, but it is not suitable for further size reduction, nor is it well suited for integration with other constituent parts of the RF front end of a portable communications device (i.e., subscriber unit). Accordingly, such a topology cannot readily be applied to such structures as antenna duplexers.
In contrast, while the devices described in U.S. Pat. No. 5,719,539 are suitable for further size reduction and system integration, such devices lack generally useful performance characteristics.
Another known method of forming a communications filter, such as a band pass filter, is to couple multiple resonant structures which reside within one or more tuned cavities. An example of such a filter is illustrated in U.S. Pat. No. 5,936,490 to Hershtig (the ""490 patent) which is directed to a filter formed with coupled tri-sections or coupled triplets. In the ""490 patent, a filter is formed with three high dielectric resonators placed within corresponding cylindrical cavities which are mutually coupled either through aperture coupling or probe coupling. Although the use of coupled triplets provides a desirable response, the construction of the filter using cylindrical cavities results in a form factor which is too large for certain applications.
Accordingly, there remains a need for filter topologies which provide high performance in a relatively compact design.
It is an objective of the present invention to provide a physically small, selective laminated filter structure which is suitable for integration within a communications system.
It is another objective of the present invention to provide a physically small, selective laminated ceramic antenna duplexer that is suitable for integration within a communications system.
It is a further objective of the present invention to provide filter topologies that produce multiple transmission zeroes with realizable element values.
In accordance with the invention a laminate dielectric filter is provided which exhibits an asymmetrical band pass response. The filter includes a dielectric laminate structure including a first dielectric layer, a second dielectric layer and a third dielectric layer. A first resonator element and a second resonator element are interposed between the first and second dielectric layers. The first and second resonator elements are arranged in a spaced apart relationship from one another and have a first end electrically connected to a circuit ground potential and a second end which is open circuited. A coupling structure is operatively coupled to each of the first resonator element and second resonator element to provide input/output ports to the filter. A third resonator element having a first end electrically connected to a circuit ground potential and a second end which is open circuited, is interposed between the second and third dielectric layers and is positioned to be disposed between the first and second resonators. A coupling element is operatively coupled to the first and second resonator elements and has a width and a position, spaced from said first ends of said first and second resonator elements, to form a coupled triplet having an asymmetrical filter wherein all transmission zeros of the response are on only one side of the filter pass band.
The resonators are preferably TEM (transverse electromagnetic) mode resonators. In one embodiment, the resonators are arranged so that the adjacent resonators form an inter-digital structure. This structure is formed by grounding the resonators on alternating ends of adjacent resonators. In this case, the coupling element is provided between the two non-adjacent resonators to produce three transmission zeroes above the pass-band. The mode of coupling between adjacent resonators is primarily electric, whereas the coupling between non-adjacent resonators is predominantly magnetic.
Alternatively, the resonators can be arranged such that all resonators are coupled to ground on adjacent ends to provide a comb-line filter section. Coupling in this type of section is primarily due to fringing fields between resonators. In this embodiment, capacitive stub pairs between adjacent resonators are provided which move the transmission zeroes below the pass-band of the structure. Each stub pair produces a transmission zero.
An additional aspect of this embodiment is the connection point of the open circuit stubs. Distributed circuits tend to have additional passbands when the length of the transmission line resonators is roughly an odd multiple of a quarter wavelength long. However, it can be seen such spurious pass bands may be suppressed if, when a line is resonant, the length from the short circuited end to the connection point is one-half wavelength, or a multiple thereof, with respect to the spurious frequency, while the electrical distance from the open circuited to connection point is an odd multiple of a one-quarter wavelength with respect to the spurious frequency. Under these conditions, the connection point of such a resonator is at a voltage null, and a series resonance is presented that short-circuits the spurious signal to ground.
In a further embodiment, the coupling element includes a transmission line whose ends overlap the non-adjacent resonators. These overlap sections act as capacitors. The addition of this transmission line element forms a structure similar to a coupled triplet filter section. The added non-adjacent coupling elements do not produce any additional zeroes but cause a frequency separation of existing zeroes in the described section.
Another embodiment of the invention provides a laminated dielectric filter which includes at least three TEM mode resonators which are arranged such that all resonators are at ground potential on adjacent ends. The structure described is a comb-line filter section. In this alternate embodiment, the coupling element takes the form of a transmission line whose ends connect to the non-adjacent resonators. The addition of this transmission line causes the coupling zeroes to shift from the low side of the pass band to the high side of the passband.
Also in accordance with the invention is a laminated dielectric antenna duplexer. The duplexer generally includes first and second coupled triplet filter sections which are cooperatively coupled with a matching structure interposed therebetween. The coupled triplet sections can take on the form of any of the filter section embodiments described herein.
One aspect of this embodiment is a juxtaposition of the constituent filters so that they share a common ground plane.
A second aspect of the present embodiment is an arrangement of the constituent filters so that all ceramic and metal layers are common.
A third aspect of the present embodiment is the matching network connecting the constituent filters.